The present invention provides compositions and methods for modulating the expression of apolipoprotein(a).
Lipoproteins are globular, micelle-like particles that consist of a non-polar core of acylglycerols and cholesteryl esters, surrounded by an amphiphilic coating consisting of protein, phospholipid and cholesterol. Lipoproteins have been classified into five broad categories on the basis of their functional and physical properties: chylomicrons (which transport dietary lipids from intestine to tissues), very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low density lipoproteins (LDL), (all of which transport triacylglycerols and cholesterol from the liver to tissues), and high density lipoproteins (HDL) (which transport endogenous cholesterol from tissues to the liver). Lipoprotein particles undergo continuous metabolic processing and have variable properties and compositions. Lipoprotein densities increase without decreasing particle diameter because the density of their outer coatings is less than that of the inner core. The protein components of lipoproteins are known as apolipoproteins. At least nine apolipoproteins are distributed in significant amounts among the various human lipoproteins.
Lipoprotein(a) (also known as Lp(a)) is a cholesterol rich particle of the pro-atherogenic LDL class. Since Lp(a) is found only in old World primates and European hedgehogs, it has been suggested that it does not play an essential role in lipid and lipoprotein metabolism. Most studies have shown that high concentrations of Lp(a) are strongly associated with increased risk of cardiovascular disease (Rainwater and Kammerer, J. Exp. Zool., 1998, 282, 54-61). These observations have stimulated numerous studies in humans and other primates to investigate the factors that control Lp(a) concentrations and physiological properties (Rainwater and Kammerer, J. Exp. Zool., 1998, 282, 54-61).
Lp(a) contains two disulfide-linked distinct proteins, apolipoprotein(a) (or ApoA) and apolipoprotein B (or ApoB) (Rainwater and Kammerer, J. Exp. Zool., 1998, 282, 54-61). Apolipoprotein(a) is a unique apolipoprotein encoded by the LPA gene which has been shown to exclusively control the physiological concentrations of Lp(a) (Rainwater and Kammerer, J. Exp. Zool., 1998, 282, 54-61). It varies in size due to interallelic differences in the number of tandemly repeated Kringle-4-encoding 5.5 kb sequences in the LPA gene (Rainwater and Kammerer, J. Exp. Zool., 1998, 282, 54-61).
Cloning of human apolipoprotein(a) in 1987 revealed homology to human plasminogen (McLean et al., Nature, 1987, 330, 132-137). The gene locus LPA encoding apolipoprotein(a) was localized to chromosome 6q26-27, in close proximity to the homologous gene for plasminogen (Frank et al., Hum. Genet., 1988, 79, 352-356).
Transgenic mice expressing human apolipoprotein(a) were found to be more susceptible than control mice to the development of lipid-staining lesions in the aorta. Consequently, apolipoprotein(a) is co-localized with lipid deposition in the artery walls (Lawn et al., Nature, 1992, 360, 670-672). As an extension of these studies, it was established that the major in vivo action of apolipoprotein(a) is inhibition of the conversion of plasminogen to plasmin which causes decreased activation of latent transforming growth factor-beta. Since transforming growth factor-beta is a negative regulator of smooth muscle cell migration and proliferation, inhibition of plasminogen activation indicates a possible mechanism for apolipoprotein(a) induction of atherosclerotic lesions (Grainger et al., Nature, 1994, 370, 460-462).
Elevated plasma levels of Lp(a), caused by increased expression of apolipoprotein(a), are associated with increased risk for atherosclerosis and its manifestations, which include hypercholesterolemia (Seed et al., N. Engl. J. Med., 1990, 322, 1494-1499), myocardial infarction (Sandkamp et al., Clin. Chem., 1990, 36, 20-23), and thrombosis (Nowak-Gottl et al., Pediatrics, 1997, 99, E11).
Moreover, the plasma concentration of Lp(a) is strongly influenced by heritable factors and is refractory to most drug and dietary manipulation (Katan and Beynen, Am. J. Epidemiol., 1987, 125, 387-399; Vessby et al., Atherosclerosis, 1982, 44, 61-71). Pharmacologic therapy of elevated Lp(a) levels has been only moderately successful and apheresis remains the most effective therapeutic modality (Hajjar and Nachman, Annu. Rev. Med., 1996, 47, 423-442).
Morishita et al. reported the use of three ribozyme oligonucleotides against apolipoprotein(a) for inhibition of apolipoprotein(a) expression in HepG2 cells (Morishita et al., Circulation, 1998, 98, 1898-1904).
U.S. Pat. No. 5,721,138 refers to nucleotide sequences encoding the human apolipoprotein(a) gene 5′-regulatory region and isolated nucleotide sequences comprising at least thirty consecutive complementary nucleotides from human apolipoprotein(a) from nucleotide positions 208 to 1448 (Lawn, 1998).
To date, investigative and therapeutic strategies aimed at inhibiting apolipoprotein(a) function have involved the previously cited use of Lp(a) apheresis and ribozyme oligonucleotides. No existing drugs are available to specifically lower lipoprotein(a) levels in humans, and only limited models exist in which to perform drug discovery. Consequently, there remains a long-felt need for additional agents and methods capable of effectively modulating, e.g., inhibiting, apolipoprotein(a) function, and particularly a need for agents capable of safe and efficacious administration to lower alipoprotein(a) levels in patients at risk for the development of coronary artery disease.